Tamoxifen, a synthetic anti-estrogen that paradoxically acts as a partial estrogen agonist on the endometrium, is associated with an increased frequency of proliferative endometrial lesions, including hyperplasias, neoplasms, and polyps. Tamoxifen-related polyps are characteristically multiple and fibrotic. A variety of metaplasias and periglandular stromal condensation may be seen. Relatively few articles have focused on the descriptive morphology of the full range of tamoxifen-associated lesions. The present study further defines the histologic features in both endometrial polyps and nonpolyp endometrium. One hundred and two specimens (including 50 polyps) were reviewed using hormone replacement therapy-related endometrial specimens and conventional polyps as the control groups. The most characteristic findings of tamoxifen-associated lesions included polarized glands along the long axis of polyps (40%), a cambium layer (72%), frequent and diverse metaplasias, staghorn glands (36%), myxoid degeneration (12%), and small glands (36%). Similar morphologic features were identified in the hormone replacement therapy and control groups but to a variable, lesser extent. Overall, the tamoxifen group consisted of 18 cases of hyperplasia (11 complex, 7 simple) and one case each of adenofibroma, adenosarcoma, endometrial stromal sarcoma, and leiomyosarcoma. Although none of the features is diagnostic, the presence of diverse metaplasias, polarized glands, staghorn glands, and a cambium layer strongly suggest tamoxifen exposure especially if a number of these features are present concurrently within the same material.